With reference to FIG. 1A and FIG. 1B, as a method of thickening a vertical wall of a bottomed container, a method in which a disk-shaped workpiece is formed into a bottomed container 11 by deep drawing, and an end surface 11b of the bottomed container is pushed by a pushing punch 14 while a bottomed container bottom surface 11a is sandwiched by a die 12 and a pressing punch 13, to thereby thicken a vertical wall 11c of the bottomed container has been conventionally in general use. In this method, however, being constrained only weakly, the vertical wall portion of the bottomed container buckles during the thickening, so that folding 11d occurs. This has made it difficult to realize a large thickening ratio (sheet thickness after the thickening/original sheet thickness) exceeding 1.3.
For example, Patent Literature 1 discloses a thickening press-work method in which a pressing punch and an upsetting punch are provided, and between an upper end portion and a bottom inner portion of a workpiece, a gap is reserved between the pressing punch and a die, and an upper surface of the end portion of the workpiece is pressed by the upsetting punch. However, in the method of Patent Literature 1, folding may also occur because buckling occurs in the gap during the thickening.
Therefore, there has been proposed a method to overcome the drawbacks of the above-described thickening methods, in the manufacture of a pulley piston for continuously variable transmission of an automobile, as described in Non Patent Literature 1. This forming method is capable of preventing folding ascribable to buckling because a portion to be thickened is thickened by pushing an end portion of a cup in a state where a constraint surface of a material is increased by providing a corner portion in the portion to be thickened. However, since a gap of the portion to be thickened is geometrically decided by constraint by the shape of a mold and the movement of the mold, a maximum thickening ratio that can be achieved in one forming is about 1.5. Further, before this process, it is necessary to form a bottomed cylindrical container by deep drawing or the like, and totally two processes are required.
Further, Patent Literature 2 proposes a method in which a press-worked workpiece which has an end wall, a bent portion bent from an outer periphery of the end wall toward one side, and a cylindrical portion provided to continue from the bent portion is used, the workpiece is clamped by a clamp punch adapted to an inner surface of the workpiece, a clamp die adapted to an outer surface of the end wall, and a thickness-increase guide die fitted to an outer peripheral surface of the clamp die so as to be swingable in an axial direction and adapted to outer peripheral surfaces of the bent portion and the cylindrical portion, a thickness-increasing punch is inserted in space formed between the clamp punch and the thickness-increase guide die to apply an axial-direction compression load to the cylindrical portion, thereby thickening the bent portion, and the thickness-increase guide die is retracted from the clamp die according to the thickening.
Further, Patent Literature 3 proposes a structure in which a knockout for receiving a body portion of a workpiece, a plate presser for pressing an area up to the middle of a rising portion rising obliquely outward from the body portion, a thickness-increasing punch for pressing a tip of the rising portion, and a die for receiving the rising portion are provided, and the die is movable in an axial direction relatively to the knockout and the thickness-increasing punch. In the process of crushing the rising portion by pressing the tip of the rising portion by the thickness-increasing punch, the die is pressed by the rising portion which is in the course of being thickened without being pushed by the thickness-increasing punch, to move relatively to the knockout, so that a gap between the plate presser and the die which sandwich the rising portion widens.
However, the methods of Patent Literatures 2, 3 also require the forming of a bottomed cylindrical container by deep drawing or the like, and thus require totally two processes. Further, a cutting process is sometimes required because a large step occurs in a boundary between a thickened portion and a non-thickened portion. Further, an achievable thickening ratio is not mentioned.
Further, in the structure in which the die is divided and part thereof is moved, in Patent Literature 2, for example, a force in a direction perpendicular to an inclined portion (2b in Patent Literature 2) acts on the thickness-increase guide die at the time of the thickening. Since this force acts in such a direction as to make the thickness-increase guide die separate from the clamp die (outward in a diameter direction), a gap occurs in a boundary between the clamp die and the thickness-increase guide die, which may cause a burr.